Thick film conductors are widely used as a means of interconnecting various passive and active devices for hybrid microcircuits and resistor networks. In this application, thick film fabrication of components has long been an economically attractive alternative to the use of thin films which involve the deposition of metal particles by vacuum evaporation or sputtering. However, relatively limited resolution capability of screened thick films has been a significant impediment to still more widespread use of thick film techniques. Typically, under the most carefully controlled circumstances, 5 mil (127 .mu.m) lines and spaces can be achieved, especially with gold conductors. In some instances, as low as 4 mil (102 .mu.m) lines and spaces can be achieved in limited quantity production when printing gold or copper thick films.
To overcome difficulties in screen printing, numerous approaches directed to providing high resolution thick film patterns have been devised. For example, the screen printing operation has been improved by such things as development of better resins for screen emulsions, improved wire mesh, stronger frames, flatter substrates and more compliant lower mass squeegee printheads. Another approach has been the development of coatings such as Prinar.RTM. to reduce the surface energy of substrates and prevent ink spreading. (Prinar.RTM. is a registered trademark of E. I. du Pont de Nemours and Company, Wilmington, DE for screen printing aids.) Also, thick film paste with improved powders, resins, solvents, flow modifiers and wetting agents have been developed. A still further approach to the problem of obtaining finer lines and spaces is (1) to apply a layer of the conductive material to a substrate by means of dispersion in a photosensitive medium, (2) to expose the layer imagewise to actinic radiation, (3) to solvent develop the pattern to remove unexposed portions of the layer, and (4) to fire the remaining exposed portions of the pattern to remove all remaining organic materials and to sinter the inorganic materials. This technique is revealed in several issued patents such as U.S. Pat. No. 3,443,944 to Wise, U.S. Pat. No. 3,615,457 to Seibert, U.S. Pat. No. 3,958.996, U.S. Pat. No. 3,982,941 to Inskip, U.S. Pat. No. 3,877,950 to Felten and U.S. Pat. No. 3,914,128 to Scheiber et al. U.S. Pat. No. 3,355,291 to Baird et al. and U.S. Pat. No. 3,573,908 to Minetti describe a method for applying glass to a semiconductor device by applying a photosensitive paste of the glass, exposing, solvent developing and firing the exposed areas.
Notwithstanding the effectiveness of the prior art processes for applying such electronic materials as a paste, it would also be advantageous to apply such materials as a dimensionally stable film. There are several advantages to the use of film, including: (1) better surface uniformity, (2) better layer thickness uniformity, (3) a thicker layer can be applied, thus taking few steps to achieve thick fired layers, (4) greater processing uniformity, (5) longer storage life, (6) minimum sensitivity to dirt pickup, and (7) avoidance of viscosity drift due to paste drying on the printing screen. Consequently, there is a strongly unmet need for conductive dispersions which can be applied by either conventional methods, such as screen printing, where appropriate, or as a laminated film where the properties named above are important.